As shown in Japanese Patent Application Laid-Open (kokai) No. H10-67335, a conventional apparatus includes a temperature sensor provided within an electric motor, and corrects drive current on the basis of temperature detected by use of the temperature sensor, to thereby compensate drop in permeability of the stator of the motor stemming from temperature increase. Further, in the publication, there is proposed a technique of estimating temperature of an electric motor through integration of drive current flowing through the motor.
The above-described technique which uses a temperature sensor incorporated in an electric motor is not preferable from the viewpoint of cost and easiness of mounting of the motor onto an apparatus which uses the motor, because of the necessity of providing a temperature sensor and connection wires for connecting the temperature sensor and a control circuit of the motor. Although the above-mentioned patent publication proposes a technique of estimating temperature of an electric motor through integration of drive current flowing through the motor, it does not describe the details of the technique.